The Panamanian firm is alleged to have set up a company in the British Virgin Islands for Mr Poroshenko called Prime Assets Partners, which describes itself as a holding company of the Roshen confectionery group. Before becoming president, Mr Poroshenko had been known as the "chocolate king".

During his presidential campaign in 2014, Mr Poroshenko vowed to sell most of his business assets. He won the election and was sworn in in June 2014. The assets were transferred to the newly created holding company.

Panama Papers - tax havens of the rich and powerful exposed

Eleven million documents held by the Panama-based law firm Mossack Fonseca have been passed to German newspaper Sueddeutsche Zeitung, which then shared them with the International Consortium of Investigative Journalists. BBC Panorama and UK newspaper the Guardian are among 107 media organisations in 78 countries which have been analysing the documents. The BBC does not know the identity of the source

They show how the company has helped clients launder money, dodge sanctions and evade tax

Mossack Fonseca says it has operated beyond reproach for 40 years and never been accused or charged with criminal wrongdoing

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In October 2014 a Ukrainian bank, of which Mr Poroshenko owns a controlling stake, sent a letter of recommendation to Mossack Fonseca saying that his accounts had been "conducted properly up to our satisfaction", according to the International Consortium of Investigative Journalists.

Anti-corruption group Transparency International also believed that the "creation of businesses while serving as president is a direct violation of the constitution".

Image copyrightAPImage caption
The company handling the sale of Roshen argued that using a foreign company was the only way to move it into a blind trust

But a spokesman for Mr Poroshenko said the company it set up for the president had no active assets and was part of a legitimate corporate restructure aimed at helping to sell the Roshen group. None of the three associated accounts held more than €2,000 (£1,600; $2,270), he said.

The Ukrainian leader went on Facebook to insist that, on becoming president, he had delegated management of his assets to consulting and law firms.

"I believe I might be the first top office official in Ukraine who treats declaring of assets, paying taxes and conflict of interest issues profoundly and seriously, in full compliance with the Ukrainian and international private law," he wrote.

Legal firm Avellum, which was given the task of selling Roshen, said that using a company with foreign jurisdiction was the only way to move the company into a blind trust. "Any allegations of tax evasion are groundless," it told Reuters news agency.

However, Oleh Lyashko, head of the populist Radical party which had earlier been part of the governing coalition, called for impeachment proceedings to be initiated, although that was unlikely to take place.